Like many classic fonts, there are several different cuts of Gill Sans by various publishers. Monotype distribute it as 'Gill Sans MT', and Adobe and ITC have their own versions.

Which version is the version that is distributed on a Mac? I can't find any authoritative answer or clues in the font files themselves. All I'm able to discern for certain is that Gill Sans (Mac) and Gill Sans MT aren't identical - at regular weight, they're almost identical (tiny, tiny differences in aliasing but no significant differences), but Gill Sans Bold as bundled with a Mac and in MT version are simply very different weights. (I currently don't have access to Gill Sans Light to test).

So, which cut of Gill Sans is it that comes with Mac OSX?

For bonus points, if it's the Monotype cut:

What are the weights of the "bold"s and "light"s in Gill Sans and Gill Sans MT? On the bold side, it looks like there's a difference of at least 200

If it's possible to complete the Mac Gill Sans set so that it matches the Gill Sans MT set by buying and installing the missing weights, is there a way of changing the namespacing so that a file created using the Gill Sans MT set imports correctly using the same-weight equivalents from the Mac set without needing to replace fonts each time?

Is the Apple one named "GillSans", without a space, rather than "Gill Sans", by any chance?
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e100Aug 13 '12 at 13:44

No, actually, it appears in lists in applications as "Gill Sans". OSX.7 Lion, I think the filename is without a space. Not-quite-in-copyright fonts get very confusing...
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user568458Aug 13 '12 at 18:40

2

Just saw Scott's answer was deleted. It contained some useful content which I'll summarise here: apparently, Apple quite often modify fonts they re-distribute in surprising ways, and it's not uncommon to delete all OS distributed fonts and replace them where needed in projects with the fonts direct from foundries to avoid quirky issues like this.
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user568458Aug 13 '12 at 18:42

Ever since Gill Sans was incorporated into the Adobe/Linotype library
in the early 1990s what used to be Monotype Gill Sans became GillSans.
The new compound name and the missing foundry attribution serves to
distance today’s users of this type from any awareness that Monotype
used to issue Gill Sans in a range of different series with alternate
cuts. Readers with experience of metal and phototypesetting may recall
this system, but for now, the majority of us only have this ‘bundled’
[with OS X] version of GillSans to go by.

Even if the "GillSans" now appears as "Gill Sans" in OS X 10.7, it seems very likely it's the same thing.

To cover your point about weight, for some reason the Bold looks to be equivalent to what Monotype or Adobe now sell as Extra Bold.

It would seem that the underlying design is common to all foundries' versions. But as yu have found, there might be small differences in hinting, or the included glyph set.

I can't answer your last point, but suspect the answer is "more trouble than it is worth"!